Scanning-type moisture detection system with sequential solid-state switching and synchronous material marking means



July 2, 1968 F. K. PREIKSCHAT SCANNING-TYPE MOISTURE DETECTION sYsTEMWITH SEQUENTIAL SOLID-STATE SWITCHING AND SYNCHRONOUS MATERIAL MARKINGMEANS Filed June 21, 1965 2 Sheets-Sheet X INVENTOR. FRITZ K ME/Ks'c/MTdrrakmsv SCANNING-TYPE MOISTURE DETECTION SYSTEM WITH SEQUENTIALSOLID-STATE SWITCHING AND SYNCHRONOUS MATERIAL MARKING MEANS Filed June21, 1965 2 Sheets-Sheet 2 y 1968 F. K. PREIKSCHAT 3,391,337

I @1 6 /6 34 L +loov F uh an m u mm r n l9 19 W 3. ATTORNEY UnitedStates Patent SCANNING-TYPE MOISTURE DETECTION SYS- TEM WITH SEQUENTIALSOLID-STATE SWITCH- ING AND SYNCHRONOUS MATERIAL MARK- ING MEANS FritzK. Preikscliat, Bellevue, Wash, assignor to Laucks Laboratories, Inc.,Bellevue, Wash., a corporation of Washington Filed June 21, 1965, Ser.No. 465,330 4 Claims. (Cl. 324--61) ABSTRACT OF THE DISCLOSURE Recurrentsequential scanning of successive zones across an advancing sheet ofwood veneer or other material in order to detect moisture content in thematerial is performed rapidly with consistent accuracy by arrayedelectrodes and respectively associated solid-state switches normallyback-biased against conduction so as to isolate the electrodes from thebridge-type detection circuit. The switches are momentarilyforward-biased in sequential order so as to connect the electrodes withthe detection circuit by suitable timing means such as a rotarymechanical switch in the bias circuit. Operated synchronously with theswitches is a separate sequentially operated selector switch meansarranged to connect individual zone markers to the detection circuit aseach zone of test material is being subjected to the measurementfunction and thereby to mark the material selectively in those zoneswherein moisture content deviates excessively from a given norm orrange. Normal back-bias of the solid-state switches affords virtuallycomplete electrical isolation of the detection circuit from the inactiveelectrodes so as to avoid admittance loading of the detection circuit.However, use of low-impedance noncritical timing means for thesolid-state switches operated synchronously with the sequential timingdevice controlling marker selection is permitted without interferencewith the critical highimpedance electrode and detection circuits, orcreating problems with switch noise, drift and erratic timing, such asproved to be unavoidable with attempts to use electromechanicalswitching of the scanned electrodes.

This invention relates to mechanism for detecting in sheet material,such as veneer, paper, etc., particular locations in which the moisturecontent exceeds a desired value by more than a predetermined amount.Such apparatus can be used to advantage in a plywood plant or in a papermill, for example.

In the past devices have been available for detecting moisture contentin sheet material in excess of a predetermined value for fairly largeareas, such as for a strip of substantial width extending across a sheetof such material. In general such indication has been accomplished byeflecting relative movement between the detecting apparatus and thesheet material in a direction lengthwise of the sheet. Where, at somelocation across the width of the sheet, the moisture content has beenexcessive or the aggregate moisture content in a strip across the widthof the sheet has been excessive, the edge of the sheet has been markedat such excessive moisture location. It has been difiicult, however, todetect with accuracy a particular moisture spot as distinguished from aband extending across such a sheet.

A principal object of the present invention is to pro- 3,391,337Patented July 2, 1968 vide a reliable moisture detector for sheetmaterial which can scan the entire surface of the sheet automatically atclosely spaced locations, and, if desired, of marking specific locationsof excess or insufficient moisture, while the moisture-detecting deviceand the sheet are moving relatively at high speed.

A further object hereof is to provide in association with an electrodeassembly electronic scanning circuit means operable to connectindividual electrode units of the assembly into a common measurementsystem recurringly in successive order through connections havingconsistently uniform and equal impedances and without introducingadmittance loading into the essentially highimpedance input of themeasurement system.

A further object is to provide such moisture-detecting and markingapparatus which will require few moving parts.

A further object is to provide such moisture-detecting apparatus whichcan detect the moisture content accurately over a considerable range ofmoisture variation.

It is also an object to provide such moisture-detecting apparatus whichwill be compact and can be incorporated conveniently in conjunction withsheet-moving means with a minimum of modification of suchsheet-conveying means.

FIGURE 1 is a top perspective of exemplary sheetconveying means withwhich the moisture detector of the present invention is associated,parts being broken away.

FIGURE 2 is a section through the moisture-detecting electrode boxinstallation on an enlarged scale.

FIGURE 3 is an electric circuit diagram of the moisture-detectingapparatus.

FIGURE 4 is a graph illustrating a moisture-detecting characteristic ofthe apparatus.

In FIGURE 1 sheet-transporting belts 1 and 2 carried and driven byrollers 3 and 4 respectively are adapted to transport the sheet materialS in the direction of the arrows. Rollers 3 and 4 are spaced apartsufiiciently to accommodate between them the elongated moisturedetection unit housing 5 positioned beneath and closely proximate to thepath of movement of the sheet material S. Bridging platforms 6 atopposite sides of the housing face span the spaces between it and thebelts 1 and 2 so that the sheet material will be backed properly betweenthe belts. The sheet material is held firmly by hold-down belts 7against the housing face and thereby against an electrode arrayincorporated therein, as hereinafter described, such belts being carriedby pulleys 8 mounted on the shafts 9. These belts are driven insynchronism with the belts 1 and 2.

As shown, the electrode assembly comprises the series array ofindividual electrode strips or segments 10 extending in a rowtransversely to the direction of movement of the sheet material S. Thesize, spacing and number of electrode strips in a given array may varyby choice, depending upon the span of the material to be scanned formoisture content, together with the size and the spacing intervalbetween individual zones or areas in which the sheet material is to betested. For example, in some applications an electrode array spanningthirteen feet is necessary, in which test intervals measuring six inchesacross the width of the material provide sufiicient resolution for testpurposes. In that case twenty-five six-inch electrode strips,successively spaced apart by a small fraction of an inch, will make upthe necessary thirteen foot span.

As shown best in FIGURE 2, the electrode array strips are mounted in anelongated aperture formed in the conductive housing Wall 34. Theaperture rim extending the length of the housing at opposite sides ofthe array serves as the opposing or ground electrode common to all thestrips. This aperture, together with the individual electrode strips,are covered and sealed by an insulation sheet 11 joined to the aperturerim. The electrode strips are connected separately through individualresistances 12 to respectively different switch contacts 13 of a rotaryswitch 14. In addition, each electrode strip is connected within theelectrode housing to a diode 15 and to a trimming condenser 16associated therewith. Each such trimming condenser may have a slottedadjustment screw 16 which is accessible through an opening 16" in theenclosure side wall so as to permit adjusting the capacitance value. Allof the diodes 15 are connected by a common conductor 15' to the commonmeasurement circuit to be described hereinafter.

Located adjacent to the discharge side of the hold-down belts 7 aresheet material marker means, shown in FIG- URE 1 as including marker dyesupply conduits 17 and 17 Each conduit supports and feeds a successionof nozzle units 18 located at intervals along its length correspondingsubstantially to the locations of the individual electrode stripswidthwise of the material sheet S. These nozzle units are individuallycontrolled by solenoid valves (not shown) in response to control signalsfrom the common measurement circuit to be described hereinafter. InFIGURE 3 the solenoid valves are designated as markers 19. For example,red dye can be supplied through conduit 17 and green dye through conduit17, with the individual markers being arranged for actuation so that ifan excessively wet spot is detected in the field of one electrode strip10 the corresponding marker nozzle solenoid associated with conduit 17will be actuated momentarily to apply a red spot to the material,whereas if an excessive dry spot is detected in the field of anelectrode segment 10 the appropriate marker nozzle solenoid associatedwith conduit 17 will be momentarily energized to apply a green mark tothe sheet at the appropriate location. Obviously, equivalent alternativeelectrode-controlled means for selective marking of the sheet may beused if desired.

Switch 14 is preferably operated continuously during movement of thesheet material on the conveyor system, such that the individualelectrode strips 10 are connected successively into the sensing andcontrol circuit in recurring cycles. The test interval lengthwise of themoving sheet is therefore determined by the relationship between sheetspeed and rotational speed of switch 14. For example, if the sheetmaterial is traveling 20 feet per second and the rotary switch 14 isturning twenty revolutions per second, the distance between the centersof locations being tested along the length of the web will be one foot.Alternatively, if the sheet is moving at a speed of four feet per secondand switch 14 is turning at a scanning speed of sixteen cycles persecond, the test interval lengthwise of the sheet will be three inches.In that event, if the width of the electrode segments parallel to thedirection of sheet travel, as shown in FIGURE 2 for example, is threeinches, the entire area of the sheet will be effectively checked formoisture content by the electrode array at this scanning speed with aresolution area of three by six inches.

In order to relate the sensing function of individual electrodes withthe marking function of individual markers, a second multiple contactrotary switch 14' is provided which is rotated synchronously with switch14 and which has contacts connected to the individual markers 19 andpositionally related to the corresponding contacts 13 of switch 14. Eachmarker 19 is thus connected to the circuit when the correspondinglylocated electrode segment is being connected to the sensing and controlcircuit.

The common measurement circuit which performs the sensing and controlfunctions of the system can be of different types but preferablycomprises a normally balanced bridge circuit 20, together withbias-controlled switching diodes 15 to connect the electrode units intothe bridge circuit momentarily in sequence. In the illustration thisbridge circuit and its associated components are in transistorized form.The bridge is energized by highfrequency oscillator 21 which iscapacity-coupled to the wiper of potentiometer R2 serving as one cornerof the bridge. Adjustment of the position of this wiper permitscompensating for any unbalance between resistances R1 and R3 connectedin the adjacent arms of the bridge serially with the respectiveresistance sections of the poten tiometer winding. The opposite cornerof the bridge is connected to ground. One intermediate corner E isperiodically connected to ground potential through the individual testelectrode segments 10 and opposing ground electrode comprising thehousing wall, such coupling being by way of common conductor 15'extending to the electrode switching diodes 15. The oppositeintermediate bridge corner D is continuously connected to groundpotential through the parallel-connected condensers 23, one of whichcomprises a trim condenser adjustable to balance the bridge with theelectrode units empty or confronting sheet material of predetermineddryness. In effect condensers 23 constitute a dummy electrode, althoughin this example ordinary condensers are used which do not provide theadvantage of electrode temperature compensation afforded in thepreferred embodiment later to be described herein.

Output from bridge 20 is derived between junction points D and E andcomprises a high-frequency voltage related in value to the admittanceeffect of moisture in the sheet material S. This output voltage iscapacitance coupled through the respective condensers C1 and C2 to thecontrol electrodes of field-effect transistors T1 and T2. Transistorbias voltage from regulated power supply 22 (energized by alternatingcurrent through terminals 23) is applied through resistances R4 and R5respectively, whereas the related ground return circuits are completedthrough the respective resistors R6 and R7. In odrer to further amplifythe output signal from bridge circuit 20 for delivery through arelatively high-current, low-impedance source such signal is fedsuccessively through transistor amplifiers T3, T4, and T5. The outputelectrode of transistor T2 is connected through condenser C4, thewinding of potentiometer R9 and resistance R11 to a positive biasterminal of power supply 22. The corresponding electrode of transistorT1 is connected through condenser C3 and the series resistances R8 andR10 to ground. One input electrode of transistor T3 is connected to thejunction between resistances R8 and R10, whereas its opposing electrodeis capacitance-coupled through condenser C5 to the base electrode oftransistor T4. Bias to transistor T3 is supplied by power supply 22through the resistance R12. The opposing input-side electrode oftransistor T4 is grounded through the parallel-connected resistance R14and capacitor C6 while its output electrode is connected to the base oftransistor T5. The latter is positively biased, as is the outputelectrode of transistor T4, through the common bias resistance R16. Thecollector of transistor T5 is connected directly to a point of positivepotential, whereas the emitter of such transistor is ground-returnedthrough resistance R15 and serves as the output of this amplifier. Suchoutput is capacitancecoupled through condenser C7 to the input ofamplifier 30.

Amplifier 30 delivers a high-frequency bridge unbalance signal to theprimary of transformer 31 whose secondary has its end terminalsconnected to opposite corners of a diode-resistance ring demodulator 26.The intermediate corners of ring demodulator 26 are energized from theend terminals of the secondary of transformer 28. The primary oftransformer 28 is energized by oscillator 21 through reference signalamplifier 27. With the center tap of transformer 23 grounded, the centertap of transformer 21 delivers a direct-voltage output signal.

This output signal is proportional in magnitude to the prevailingunbalance of the bridge circuit, hence to the moisture loading of theindividual test electrode segment which is then connected in the bridgecircuit. This direct-voltage signal may be recorded in the unit 32. asshown and may also be applied to the level selector 3 3 whose functionwill be described.

For reasons previously mentioned, bias-controlled switching diodes areused to connect the individual electrodes 10' into the commonmeasurement bridge circuit momentarily in sequence and to isolate themfrom the circuit at all other times. Thus, each electrode segment 10 isconnected to conductor 15 through an individual diode 15 which isnormally back-biased against conduction by connecting its anode toground through a resistance 12 and a second resistance in seriestherewith, and by connecting its cathode through conductor 15' to asource of positive bias. This positive bias potential is delivered forconvenience with bridge junction E. An adjustable trim condenser 16 isconnected between each individual electrode segment and ground in orderto equalize its normal admittance with those of all other electrodes. Aswill be noted, the anode" of each diode is connected through resistance12 to a different stationary contact 13 of switch 14. The power-drivenrotor of this switch 14 is connected through conductor 35 to a point ofhigh potential in the output of power supply 22, such potential beingsuificiently high that its application to the anode of a diode 15 willproduce conductivity in such diode and thereby connect the associatedelectrode segment 10 to the bridge circiut input junction E. Thus, asthe switch 14 rotates (which it normally does at constant speed duringsystem operation) the electrodes are successively connected by theirrespective diodes 15 into the bridge circuit and at all other times areisolated therefrom by the applied back-bias. Capacitance coupling of theoscillator 21 to the bridge circuit and capacitance coupling of thebridge terminals D and E to the amplifiers T1 and T2 serves to permitvoltage controlled switching of diodes 10' without disturbing voltageson transistors T1 and T2 and without permitting operating bias in thebridge circuit from affecting the diodes.

Diodes 15 are of the solid-state type (i.e., germanium, etc). As such,they introduce only a very small amount of capacitance into the circiutand while, as shown, they may be controlled by a mechanical switch, theyexhibit substantially unvarying conduction impedance as compared withmechanical switch contacts. These attributes are of considerableimportance to consistently reliable and uniform moisture measurements inall areas of the sheet material when it is realized that the range ofmoisture content in such materials changes the electrode capacitance byonly a few hundredths of a picofarad. As described in co-jendingapplication Ser. No. 531,207 filed Mar. 2, 1966, a thermistor 24,connected via a shielded conductor 25 to the output of the bridgecircuit may be used to compensate for the elfects of temperature changein the material itself which otherwise also greatly affects therelationship between moisture content and apparent electrode admittance.

As previously mentioned, switch 14' rotates synchronously with switch 14so that as each individual electrode strip 10 is being connectedmomentarily into the bridge circuit, the associated marker 19 is beingconnected momentarily by switch 14' to the output of level selector 33.This level selector comprises a threshold circuit of any suitable orknown type which permits actuation of an individual marker 19 when thelevel of the output signal from ring demodulator 26 exceeds a certainvalue. Thus minor and inconsequential fluctuations in the balance ofbridge circuit 20 do not actuate the markers. It will be evident that aseparate set of markers or a separate and additional rotary switchequivalent to switch 14 may be employed if desired, such that unbalanceof the bridge in the opposite sense may also be detected and utilized toapply marks to the sheet material. Thus, the specific arrangement shownin FIG- URE 3 is capable of marking the sheet material in the case ofexcess moisture, for example, whereas by the provision of an additionallevel selector energized by the same or an additional ring demodulatorand feeding a separate sequencing switch equivalent to switch 14' thesheet material may also be marked for areas of insufiicient moisturecontent.

FIGURE 4 illustrates a typical response characteristic of the circuit interms of output voltage as a function of material moisture content. Fromthis graph it will be recognized that the circuit may be designed andadjusted so that the output voltage becomes positive when moistureexceeds a preselected level such as 5 percent, and negative when itdrops below that moisture level. Initial adjustment of the bridgecircuit to determine the zero condition may be achieved by appropriatesetting of the trim condense-r in the dummy electrode unit 23.

I claim as my invention:

1. Material scanning type moisture detection apparatus comprising anarray of similar tes-t electrodes and associated electrode meansarranged in electrical relationship with the test material, a capacitiveimpedance measurement circuit common to said electrodes and electrodemeans, and means for connecting the individual test electrodes andassociated electrode means in capacitive impedance measurementrelationship successively to the measurement circuit, includingbias-controlled solid-state switches individual to the respective testelectrodes and commonly housed for uniformity of thermal environment,means normally back-biasing said switches against conductivity, andmeans for momentarily forward-biasing said switches in sequence torender the same conductive and thereby connect said test electrodes andassociated electrode means in capacitive impedance measurementrelationship to said circuit sequentially to scan the test material.

2. The apparatus defined in claim 1, wherein the means for connectingthe individual electrodes comprising a rotary mechanical switching meanshaving contacts successively connected to forward-bias the solid-stateswitches in sequential recurring cycles.

3. The apparatus defined in claim 2, and a plurality of indicating meanscorresponding to the respective electrode-s, and indicating switch meansoperable synchronously with the mechanical switching means and arrangedto responsively connect the individual indicator means to themeasurement circuit.

4. Material scanning type moisture detection apparatus comprising anarray of similar test electrodes and associated electrode means arrangedin electrical relationship with the test material, a capacitiveimpedance measurement circuit common to said electrodes and electrodemeans, means for connecting the individual electrodes and associatedelectrode means in capacitive impedance measurement relationshipsuccessively to the measurement circuit, including bias-controlledsolid-state switches individual to the respective test electrodes andcommonly housed for uniformity of thermal environment, means normallyback-biasing said switches against conductivity, means for momentarilyforward-biasing said.

switches in sequence to render the same conductive and thereby connectsaid test electrodes and associated electrode means in capacitiveimpedance measurement relationship to said circuit sequentially to scanthe test material, a plurality of markers operatively associated withthe respective test electrodes and individually operable to mark thetest material selectively at locations thereon electrically related tothe respective test electrodes, means for operating said markers inresponse to predetermined measurement circuit response includingselector switch means for operatively connecting said measurementcircuit to the individual markers sequentially in timed relation withthe sequential momentary forward-biasing of markers.

References Cited 736,464 9/1955 Great Britain. UNITED STATES PATENTSFritZinger X 5 Primary Examiner. 2/ 1957 Dallas 32461 ARCHIE R.BORCHELT, Examiner. 4/1960 Gootherts 340-176 X 6/1960 Shawhan XR E. E.KUBASIEWICZ, Asszstant Exammer. 12/1960 Shawhan 32461 XR FOREIGN PATENTS673,684 11/1963 Canada.

